This invention relates to a winding shaft for a strip of sheet material, and more particularly to a winding shaft of the type provided on the circular periphery thereof with rollers for fixing a spool in position thereon.
In the conventional device in which a strip of sheet material is wound on a spool fixed round a winding shaft, blades are attached in the axial direction to the winding shaft in such a way that the spool may be slid over the winding shaft and removed therefrom and the spool, once fixed in position on the winding shaft, may be rotated in conjunction with the winding shaft. This device has the disadvantage that the inner surface of the spool is injured and the attachment and detachment of the spool to and from the winding shaft are not necessarily effected smoothly.
Recently, a new device in which instead of the blades on the winding shaft, a multiplicity of frictional lugs are pneumatically forced out of the shaft interior so as to protrude from the circular periphery of the shaft and come into forced contact with the inner surface of the spool and impart frictional drive to the spool is finding increasing acceptance.
This so-called "air shaft" indeed enjoys the advantage that it finds no use for blades. Because of the requirement that the aforementioned frictional lugs should be radially forced out of the interior of the winding shaft, however, the winding shaft is complicated in structure, expensive to manufacture, and prone to no few causes for mechanical trouble.
In earlier days, a device in which one groove is formed axially on the circular periphery of a winding shaft throughout the entire length of the shaft and a long bar is inserted into the groove after a spool has been slid over the winding shaft and an improved version of this device in which the long bar is inserted in the groove inseparable in advance of the attachment of the spool to the winding shaft were prevalent. Since these devices both have one long bar wedged into the gap between the winding shaft and the spool, the winding shaft and the spool cannot be retained coaxially and the spool is locally deformed. In either of these earlier devices, the rotation of the winding shaft about its axis causes the spool to produce an eccentric rotation, which constitutes a fatal drawback to the recent high-speed winding operation. Further, when a round bar is inserted as a wedge, since the length of the round bar is so great that the pressure with which the round bar is held in contact with the inner surface of the spool may not be uniform throughout the entire length of the round bar, there is a possibility that only one end of the round bar will force its way into the inner surface of the spool and the other end portion thereof will fail to produce a locking action as expected. When the length of the roller inserted in the groove is appreciably smaller than that of the groove, the portions of the wall of the spool falling directly above the portions of the groove empty of the roller tend to cave in under the pressure exerted by the strip of sheet material being wound on the spool. The device using such a short bar is hardly suitable for accurate winding. Since these problems still remain unsolved, the devices of this principle find no general acceptance today.